The file extension _PDF is created when Internet Explorer or other applications mysteriously change the file extension when you download documents from the Internet. This process is commonly the result of a bug in the application and prevents you from opening files in Portable Document Format unless you change the extension back to PDF.
The issue with _PDF files commonly occurs when you use Internet Explorer to browse content on the MobileMe website. Internet Explorer changes file names especially when you are using a file sharing tool. When the file names are changed, spaces are placed in the file extension at random or as an underscore such as _PDF which appears in front of the file extension.
How to Fix File Extension _PDF
You can go about fixing the issue with the _PDF extension by opening up Internet Explorer and then clicking on Tools on the main toolbar. On the drop-down menu, click on Internet Options and then choose the Security tab.
Under the Security tab select Trusted Sites and then click on Sites. Remove the check mark in the box next to Require server verification (https) for all sites in this zone. In the prompt, add the URL for the website where you are trying to access PDF documents such as MobileME.com.
When you have completed the process, click on Close to exit the window and then click on OK. Close the Internet Explorer browser, wait a few seconds, and then reopen Internet Explorer and try downloading the PDF document again.
Fixing  File Extension _PDF  in the Registry Editor
If you are tech savvy and understand how your PC’s registry works, then you can try this remedy for _PDF file extension. We must emphasize that if you do not understand the registry, do not attempt this method since the registry is responsible for all operations on your PC. One mistake and you may not be able to boot your PC or other serious errors can result from making a wrong move with the registry.
That said you can you can work your way down in the registry to the Main and Feature Control for Internet Explorer under H_KEY_LOCAL_MACHINE. The path under H_KEY_LOCAL_MACHINE is as follows: Softwareà Microsoftà Internet Explorerà Main and Feature Control.
Create a new registry key by clicking on Edit and choosing New from the drop-down menu. Click on Key and then enter FEATURE_PRESERVE_SPACES_IN_FILENAMES_KB952730. Then create a new registry value by clicking on Edit and choosing New on the menu and selecting DWORD Value. Enter iexplore.exe and then hit the Enter key. In the new window, click on Edit on the main toolbar again and choose Modify and enter 1. Click on OK and close the registry.
